The Effect of Cu Target Pad Roughness and Solution Flow on the Growth Mode and Void Formation in Electroless Cu Films

The effect of the Cu target pad roughness on the growth mode of electroless Cu from two different Cu baths was investigated, with bath A having a cyanide based, and bath B, a non-cyanide-based stabilizer system. Both baths are commonly used within the PCB industry. In the case of bath B, for an average target pad roughness higher than Ra = 300 nm, two growth modes are observed. The first mode is a copying of the subjacent Cu substrate morphology, whereas the second forms spherical grains (Cu-nodules) predominantly at the exposed sites of the substrate crystals. These Cu nodules typically have a radius comparable to that of the plated electroless Cu thickness and contain a high density of nanovoids toward their base. The related void formation seems relevant to weaken the overall Cu/Cu/Cu interconnection in the blind microvia. Interestingly, the tendency to form nodules with increasing Cu base roughness is widely suppressed for the cyanide-based bath A, where the deposit is nodule free up to a target pad roughness of approximately Ra=1,000 nm. When solution delivery and exchange were investigated, it is apparent that a low exchange rate has a negative impact on the electroless Cu deposition, and results with undesirable nodules and voids, even if the surface roughness values would suggest otherwise, could be expected.